Semiconductor diode lasers with broad area active regions often produce multiple independent light filaments in the laser cavity. Such filaments are formed as the light builds up in the laser cavity. That is, in ordinary broad area lasers, the light path comes back directly over its prior path. As the light intensity along this path increases, the free carrier density at the center of the filament is somewhat decreased. This decrease in charge density causes an increase in the refractive index at the center of the mode, thereby leading to a stable waveguide for the filament. Unfortunately, each filament is stable within itself and is not coherent with other regions of the broad area laser. Therefore, any broad area laser which laser with multiple independent filaments is incoherent and multimode.
In U.S. Pat. No. 3,969,686, Scifres et al. describe a diode laser in which all four sides of the semiconductor material are cleaved to provide a low loss, totally internally reflecting light path within the active medium of the laser. In order to control which portions of the active medium lase so as to control the path that the totally internally reflected light travels, striped current confinement geometries are used to restrict gain to two spaced parallel elongated regions at 45.degree. to the cleaved surfaces. A periodic structure within the laser having a plurality of parallel teeth is oriented such that the teeth are perpendicular to two sections of the light path, thereby providing distributed feedback for coherent light amplification. The teeth make a 45.degree. angle to the cleaved surfaces. The teeth of the periodic structure are spaced an integer number of wavelengths apart, so that they provide two parallel output beams which are perpendicular to the plane of the pn junction.
In U.S. Pat. No. 4,112,389, Streifer et al. describe a diode laser having an active region with a ring-shaped section and a coupler section coupled to a waveguide. The laser will oscillate at a wavelength .lambda. that corresponds to a path length around the ring of p.lambda.+.lambda./2, where p is an integer, since, at such a wavelength, a lightwave beginning at the coupler section and traveling in either direction around the ring will be 180.degree. out-of-phase with a lightwave that has not yet circulated around the ring and will destructively interfere so that transmission through the coupler section into the waveguide is at a minimum. The degree of coupler symmetry in a given design may be chosen to provide the desired amount of coupling to the waveguide. Semiconductor material is absent above the substrate in all areas not corresponding to the ring-shaped section, coupler section or waveguide, in order to produce mesa-type waveguiding in the active region.
In U.S. Pat. No. 4,792,962, Miyauchi et al. describe a semiconductor laser having a square-shaped optical waveguide with diagonal reflecting mirrors at each of the four corners of the optical waveguide formed from the four cleaved facets of the semiconductor device. A curved waveguide is disposed adjacent to one of the four sides of the square-shaped waveguide for releasing light by means of evanescent optical coupling from the ring-shaped resonant cavity constituted by the square-shaped waveguide. The ends of curved waveguide intersect two of the facets at approximately right angles, through which the laser light is output.
In U.S. Pat. No. 4,924,476, Behfar-Rad et al. describe a semiconductor ring laser having a triangular cavity with three longitudinal cavity sections integrally formed on a substrate, the three sections being joined at their ends to form three apices at which light reflective facets are positioned. Two of the facets are oriented to provide total internal reflection, while the third has a preselected angle for incident light which is less than the critical angle for total internal reflection, so that it is partially transmissive. The preselected angle determines that facet's reflectivity. Facets are formed through the use of chemically assisted ion beam etching.
In the article "Two-dimensional distributed-feedback lasers and their applications" in Applied Physics Letters., vol. 22, no. 9, 1 May 1973, pp. 460-462, S. Wang et al. describe a laser waveguide structure applicable to semiconductor lasers and dye-impregnated thin-film lasers in which the thickness of the waveguide structure varies periodically in two directions. That is, a two-dimensional grating is formed adjacent to the waveguide layer or film to form a two-dimensional distributed-feedback laser. The authors note that one characteristic of such a laser is that the output will show multiple modes oscillating simultaneously, because of the availability of many reciprocal-lattice vectors corresponding to the periodic grating structure on the waveguide. Each laser mode shows a very narrow spectral width and a sharply defined direction.
In the article "Optically Pumped GaAs Lasers with Two-Dimensional Bragg Reflectors" in IEEE Journal of Quantum Electronics, vol. QE-13, no. 10, Oct. 1977, pp. 806-807, T. Kawamura et al. describe a two-dimensional distributed-feedback laser in which a two-dimensional ring mode and a one-dimensional DBR mode oscillated simultaneously, because of the availability of several reciprocal lattice vectors corresponding to the periodic grating structures within the gain bandwidth of the lasing medium.
Objects of the present invention include:
(1) providing a broad area semiconductor laser in which mutually incoherent light filaments are prevented from developing in the laser cavity, whereby the laser operates in a stable nonfilamentary mode and emits a coherent output beam; PA0 (2) providing broad area lasers capable of operating in a single transverse mode, a single longitudinal mode (single frequency) or both, even at high output power levels; PA0 (3) providing wavelength tunable, broad area lasers; PA0 (4) providing semiconductor ring lasers having a unidirectional ring mode; PA0 (5) providing lasers which can be amplitude or frequency modulated at high speed and with low power consumption; PA0 (6) providing lasers with a steerable output beam; PA0 (7) providing master oscillator power amplifier (MOPA) devices, based on any of the aforementioned broad area laser oscillators, that emit a coherent output beam; and PA0 (8) providing a low divergence (less than 1.degree. by 1.degree.), grating coupled surface emitter based on any of the aforementioned lasers or MOPA devices.